1. Field of the Invention
The present invention relates to a high-power semiconductor light emitting diode (referred to hereinafter as "LED") display device, and particularly, to a LED display device having a LED matrix circuit for an outdoor/indoor display panel, a railway sign board, a traffic sign board, or a vehicle-mounted light.
2. Description of the Prior Art
Semiconductor light emitting devices such as LEDs and semiconductor lasers are manufactured according to a liquid-phase epitaxial growth (LPE) technique and a vapor-phase epitaxial (VPE) growth technique such as a metal organic chemical vapor deposition (MOCVD) technique. Any of the techniques forms a double hetero (DH) structure to confine carriers in an active layer serving as a light emitting layer and realize high brightness.
FIG. 1 is a sectional view showing an InGaAlP-based LED having a DH structure according to a prior art. Successively laminated on an n.sup.+ -type GaAs substrate 1 are an n-type GaAs buffer layer 2, an n-type In.sub.0.5 Al.sub.0.5 P/GaAs reflection layer (quarter-wave stack mirror) 3, an n-type In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P clad layer 41, an undoped In.sub.0.5 (Ga.sub.1-y Al.sub.y).sub.0.5 P active layer 61, and a p-type In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P clad layer 81. Here, x.ltoreq.1 and x&gt;y.
On a part of the clad layer 81, an n-type GaAs current block layer 91 is formed. On the current block layer 91 and clad layer 81, a p-type Ga.sub.0.3 Al.sub.0.7 As current diffusion layer 10 is formed. On a part of the current diffusion layer 10, a p.sup.+ -type GaAs contact layer 11 is formed. On the contact layer 11, a p-type electrode 13 is formed. On the bottom surface of the substrate 1, an n-type electrode 12 is formed.
The structure of FIG. 1 is epitaxially grown according to a low pressure MOCVD (LPMOCVD) technique that employs trimethylindium (TMI), trimethylgallium (TMG), and trimethylaluminum (TMA) as materials of group III, arsine (AsH.sub.3) and phosphine (PH.sub.3) as materials of group V, silane (SiH.sub.4) and dimethylzinc (DMZ) as doping materials, and hydrogen as a carrier gas. These materials epitaxially grow crystals under a low pressure. More precisely, a wafer having an n-type GaAs substrate 1 is placed in a CVD reactor and is kept at a given pressure and temperature. A mass flow controller supplies the group III, V, and doping materials into the reactor at set flow rates, to epitaxially grow layers one after another on the substrate 1. Namely, an n-type GaAs buffer layer 2, an n-type In.sub.0.5 Al.sub.0.5 P/GaAs reflection layer 3, an n-type In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P clad layer 41, an undoped In.sub.0.5 (Ga.sub.1-y Al.sub.y).sub.0.5 P active layer 61, a p-type In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P clad layer 81, and an n-type GaAs current block layer 91 are successively formed on the substrate 1. Here, x.ltoreq.1 and x&gt;y.
The wafer with the layers is taken out of the reactor, and the current block layer 91 is selectively etched according to a photolithography technique into the shape of FIG. 1. The MOCVD method is again employed to form a p-type Ga.sub.0.3 Al.sub.0.7 As current diffusion layer 10 and a p--type GaAs contact layer 11.
Au-based material is deposited on each surface of the wafer according to a vacuum evaporation technique. The Au-based layer is selectively etched according to the photolithography technique to form a p-type electrode 13 as shown in FIG. 1. The contact layer 11 is selectively etched to partly expose the current diffusion layer 10. An n-type electrode 12 is formed over the bottom surface of the substrate 1. The wafer Is diced into chips that serve each as the semiconductor light emitting device of FIG. 1. Each chip is mounted on a stem, bonded, sealed with resin, and fabricated into a .phi. 5-mm lamp.
FIG. 2 is a graph showing the initial brightness I.sub.o and remnant brightness ratio .eta. of the .phi. 5-mm lamp. The remnant brightness ratio is the ratio of the brightness I.sub.1000 of the lamp measured after 1000 hours of operation at 50 mA to the initial brightness I.sub.o. Namely, .eta.=(I.sub.1000 /I.sub.o).times.100. Here, the mole fraction "y" of Al of the active layer 61 is 0.3, and the mole fraction "x" of Al of each of the clad layers 41 and 81 is changed among 1.0, 0.9, 0.8, and 0.7.
When the Al mole fraction "x" of In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P of the clad layers 41 and 81 is increased, the initial brightness I.sub.o increases but the remnant brightness ratio .eta. decreases. When the Al mole fraction "x" is decreased, the initial brightness I.sub.o decreases but the remnant brightness ratio .eta. increases. In this way, the initial brightness I.sub.o and remnant brightness ratio .eta. are trade-offs. It is difficult for the conventional DH structure to provide high brightness as well as long service life.
FIG. 3 shows an InGaAlP-based LED having a DH structure according to another prior art. Sequentially laminated on an n.sup.+ type GaAs substrate 1 are an n-type GaAs buffer layer 2, an n-type In.sub.0.5 Al.sub.0.5 P/GaAs reflective multilayer 3, an n-type In.sub.0.5 Al.sub.0.5 P clad layer 42, an n-type In.sub.0.5 (Ga.sub.0.72 Al.sub.0.28).sub.0.5 P active layer 62, a p-type In.sub.0.5 Al.sub.0.5 P clad layer 82, a p-type In.sub.0.5 Ga.sub.0.5 P contact layer 127, and a p-type GaAs protection layer 128. On a part of the protection layer 128, there are sequentially laminated an n-type In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P current block layer 92, an n-type GaAs protection layer 93, a p-type Ga.sub.0.3 Al.sub.0.7 As current diffusion layer 10, a p-type In.sub.0.5 (Ga.sub.0.7 Al.sub.0.3).sub.0.5 P diffusion layer 132, and a p.sup.+ -type GaAs contact layer 11. The contact layer 11 is formed on a part of the diffusion layer 132. On the contact layer 11, a p-type electrode 13 is formed. An n-type electrode 12 is formed on the bottom surface of the substrate 1.
When designing the clad layers 42 and 82 that sandwich the active layer 62 serving as a light emitting layer, the following opposing factors must be considered:
(a) To confine a sufficient quantity of minority carriers in the active layer 62, the band gap (Eg) of the clad layers 42 and 82 must be properly larger than that of the active layer 62. Namely, the Al mole fraction "x" of the clad layers 42 and 82 must be large.
(b) Crystal defects that trap minority carriers must be minimized in each interface between the clad layers 42 and 82 and the active layer 62. Such defects, however, easily occur in the interfaces where the Al mole fraction of the clad layers 42 and 82 greatly differs from that of the active layer 62. To reduce the crystal defects, the Al mole fraction "x" of the clad layers 42 and 82 must be small.
When the Al mole fraction of the clad layers 42 and 82 greatly differs from that of the active layer 62 that emits yellow light, initial brightness may be high but there will be many crystal defects in each interface among the layers. As a result, normalized light intensity P/P.sub.o deteriorates in proportion to an operating time as shown in FIG. 4.
The Al mole fraction of the clad layers of the conventional DH structure LED must be high to provide high brightness when the LED is used outdoors. This, however, causes many crystal defects to shorten the service life of the LED. If the Al mole fraction of the clad layers is low to extend the service life of the LED, the light output of the LED will be low, and therefore, the brightness thereof will be improper for outdoor use. In this way, the light output and service life of the conventional LED are trade-offs.
The conventional DH structure semiconductor light emitting devices are incapable of simultaneously providing high brightness and long service life.